The present invention relates to the monitoring of selected operating characteristics of a furnace, the display of these characteristics and the use of these characteristics in the optimization and control of a furnace. More specifically, the present invention relates to the monitoring, display, optimization and control of the performance of a kraft process chemical recovery furnace of the type in which black liquor fuel is introduced and burned to produce a smelt bed at a lower region of the furnace.
The monitoring of a hot infrared emitting surface obscured by particulate fume and hot gases, such as found in kraft pulp recovery boilers, is a difficult task. That is, interference from fume particles and gaseous radiation within the furnace tends to obscure the view of hot surfaces, such as of the smelt bed and background, under such adverse environmental conditions.
U.S. Pat. No. 4,539,588 to Ariessohn, et al. describes one form of an apparatus for this purpose. In particular, the Ariessohn, et al. device comprises a closed circuit video camera fitted with an infrared imaging detector or vidicon tube. An objective lens obtains the image. An optical filter interposed between the lens and vidicon is selected to reject radiation in all but limited ranges of radiation to avoid interference by gaseous species overlaying the smelt bed, such gases being strongly emitting and absorbing. As a specific example, a spectral filter centered at 1.65 .mu.m with a band width of 0.3 .mu.m is noted as being suitable for imaging a kraft recovery smelt bed.
A product known as TIPS.TM. from the Sensor and Simulation Products Division of Weyerhaeuser Company of Tacoma, Wash., incorporates the device of the Ariessohn, et al. patent in a temperature image processing and storage system. The TIPS.TM. system creates digitally colorized images of the smelt bed for viewing by an operator. In the TIPS.TM. system, due to the partial elimination of the effects of moving particles in the image, the view of active scenes on the bed is permitted. The TIPS.TM. system is especially designed for displaying temperature trends of the bed on digital and graphic displays and for tracking changes from a reference temperature at a selected location in the process, or to observe temperature differences between locations. In addition, the TIPS.TM. system allows the production and storage of historical temperature changes. Moreover, the TIPS.TM. system permits the manual adjustment of a reference temperature for purposes of comparison.
The TIPS.TM. system has a number of advantages, but also suffers from limitations. The TIPS.TM. system provides limited temperature information concerning the smelt bed. That is, the pyrometer is utilized to determine a temperature over a small calibration window. An observation window, representing about 2-3 percent of the digital image displayed on a screen, is then moved to a desired location on the screen with the temperature then being determined within that observation window relative to the temperature in the calibration window. In general, the intensity of the signal in the calibration window is known, and the temperature in the calibration window is known from the pyrometer (subject to errors as mentioned above). Therefore, the temperature in the observation window can be inferred by comparing the intensity of the image in the observation window to the intensity of the image in the calibration window. In this way, limited spot temperature information at various locations throughout the image can be obtained by shifting the small observation window. Also, in the TIPS.TM. system, a pyrometer is used with a field of view which is separate from the field of view of the video camera. Consequently, errors can be introduced into this system due to the difficulty in precisely matching the location of the field of view of the pyrometer to the location in the field of view of the camera for calibration purposes.
The capabilities of the TIPS.TM. system are described in greater detail in an article published in April 1989 entitled "Monitoring of Recovery Boiler Interiors Using Imaging Technology," by Anderson, et al. (CPPA-TAPPI 1989, International Chemical Recovery Conference). In addition to discussing the imaging of a bed for purposes of developing temperature trend information, this particular article mentions that adequate smelt reduction requires sufficient bed residence time, which is influenced by bed configuration. The article also recites that both of these issues can be addressed by a bed-level monitoring system which can extract the bed profile and alert the operators when the bed drifts out of the user-defined range. The article then mentions that the Weyerhaeuser (TIPS.TM. ) system has the capability to detect bed height so as to provide a control signal for those interested in using bed height or slope for control purposes. However, this article does not provide any information on how these goals would be accomplished.
U.S. Pat. No. 4,737,844 to Kohola, et al. describes a system utilizing a video camera for obtaining a video signal which is digitized and filtered temporally and spatially. The digitized video signal is divided into signal subareas with feature elements belonging to the same subarea being combined into continuous image areas corresponding to a certain signal level. The combined subareas are then processed to provide an integrated image which is averaged to eliminate the effect of random disturbances. The averaged image is displayed on a display device. The images may then be compared to optimum conditions. Areas corresponding to effective combustion and the flame front of a bed are then defined, using histograms, and identified by means of their area, point of gravity coordinates of the area and point-by-point recorded contours of the area. In addition, the contours of voids inside the area are defined. In an application described in the Kohola, et al. patent, the flame front, location and shape of the fuel bed is determined.
In Kohola, et al., the material to be burned is shown as a bed of substantially identical thickness and width. This bed is delivered to the mill end of a boiler stoker where the flame front is concentrated. Thus, Kohola, et al. is described in conjunction with a bed of a substantially uniform contour and is not directed toward beds such as are found in smelt bed boilers which are burning throughout substantially their entire surface and wherein the contours of the bed vary depending upon furnace operating parameters, such as the fuel-to-air ratio.
In U.S. Pat. No. 5,139,412, to Kychakoff, et al. and entitled "Method and Apparatus for Profiling the Bed of a Furnace," the determination of characteristics of the shape and volume of a smelt bed of a black liquor furnace is described. This patent application is hereby incorporated by reference herein. In accordance with this description, a digital image of the bed and background is produced. The digital image is then processed to determine transitions in the image which correspond to transitions between the bed and background and thereby to the profile and boundary of the bed. The determination of bed characteristics comprising the bed profile, bed height, slope and the volume of the bed is disclosed. These characteristics are displayed, or otherwise used, for example, in the control of the parameters affecting the operation of the furnace, such as in controlling the air-to-fuel ratio in the furnace.
The Kychakoff, et al. disclosure also mentions the provision of a reference bed characteristic and the comparison of the determined bed characteristic with the reference bed characteristic. In the event of a difference in excess of a threshold, an alarm or other indicator is activated. Alternatively, use of the determined bed characteristics in the automatic control of a furnace, and in particular air-to-fuel ratios, is described. Histories of these characteristics may be stored and correlated to furnace performance characteristics, such as fuel efficiency, reduction efficiency and the like, for use in developing a target bed configuration which optimizes these conditions. The furnace is then operated to provide a determined bed which matches the target bed.
In accordance with the Kychakoff disclosure, digital images of the bed are obtained and processed to determine transitions indicative of the boundary of the bed. The processing approach described in this patent includes the steps of selecting images from the plural digital images for clarity; temporally averaging the selected images; differentiating the images following temporal averaging; smoothing the images; and thereafter locating transitions in the images. The step of locating transitions is described as including the performance of a continuity check and/or a region growing process.
Although this Kychakoff, et al. invention provides a desirable approach for determining bed profiles, there is nevertheless a need for improvements in bed characteristic determination. In addition, this Kychakoff, et al. invention does not recognize the complex interaction between furnace operating parameters and characteristics other than those associated with the bed profile.
The problem of carryover particles in kraft chemical recovery boilers has heretofore been recognized. In general, carryover particles may be defined as "out-of-place" burning particles that are traveling in a furnace or boiler in a region well above the hearth of the furnace. More specifically, carryover particles in smelt bed recovery boilers may be defined as the mass of burning or hot smelt particles passing a horizontal plane at an upper level of the boiler, such as at the "bull nose" level within the boiler. Burning particles which encounter steam tubes in such a recovery boiler are quenched and form hard deposits on the tubing. These hard deposits are difficult to clean or remove through the use of typical steam cleaning mechanisms in such boilers. These particles typically contain sodium sulfate and sodium carbonate, but may also include other components to a varying extent, such as residual organics from the black liquor.
Devices for detecting carryover particles in the interior of furnaces, such as kraft process chemical recovery furnaces, are known. One such device is disclosed in U.S. Pat. No. 5,010,827 to Kychakoff, et al., which is incorporated by reference herein. This Kychakoff, et al. device utilizes plural spaced apart detectors for monitoring discrete portions of the interior of a furnace for the purpose of detecting carryover particles at such monitored locations. Signals indicative of the carryover particles are processed to obtain a count of the carryover particles. The carryover particle count may then be displayed. For example, the signals from all of the detectors may be averaged with trends and overall changes in count rates then displayed. In addition, the counts from the individual detectors may be displayed to assist an operator in locating the source of excessive carryover particles in the furnace. The information on carryover particle count may be used in controlling parameters affecting the performance of the furnace directly or indirectly by way of operator input. This patent specifically mentions that under certain boiler or furnace conditions, such as resulting from disturbances in the air supply or perhaps due to a high bed volume in the boiler, carryover particle increases may occur. The control of air and fuel flow in response to carryover particle count is also specifically mentioned in this patent.
Although the invention of U.S. Pat. No. 5,010,827 offers a number of advantages, this patent does not recognize the importance of simultaneously monitoring characteristics of a furnace in addition to carryover particles. U.S. Pat. Nos. 4,690,634 to Herngren, et al., and 4,814,868 to James also relate to the monitoring of carryover particles in boilers. U.S. Pat. No. 3,830,969 to Hofstein describes yet another system for detecting particles. These latter systems suffer from limitations in their ability to accurately detect carryover particles.
Although systems exist for use in monitoring the interior of recovery boilers, a need exists for an overall improved system for simultaneously monitoring temperature, bed profile and carryover particles in such furnaces. In addition, improvements in bed profile and temperature determinations are also highly desirable. In addition, a need exists for a method and apparatus for monitoring plural furnace operating characteristics and which facilitates the display of this information for use in monitoring, optimizing and controlling the operation of a kraft process chemical recovery furnace.